Never-before-seen footage has revealed a striking natural event in southern Brazil: thousands of bumblebee catfish climbing a waterfall in coordinated fashion. The spectacle, recently documented in the Journal of Fish Biology, marks the first detailed observation of this behavior in the species Rhyacoglanis paranensis. Scientists believe the phenomenon is linked to reproduction and the changing dynamics of Brazilian river ecosystems, shedding new light on a little-known freshwater species.
Rare Glimpse Into A Hidden Species
The bumblebee catfish is a small, vividly patterned fish measuring only about 3.5 inches. It is named for its orange body marked with distinct black spots. Belonging to the genus Rhyacoglanis, these catfish are distributed across the Amazon, Orinoco, and La Plata river basins but are rarely observed in large numbers. In November 2024, during the onset of the rainy season, members of Brazil’s Environmental Military Police first spotted thousands of them clinging to the slippery rocks of the Aquidauana River waterfall.
This observation was extraordinary because the genus is scarcely seen in the wild. “[This genus] is rarely collected and even less frequently observed,” explained Manoela Marinho, a biologist at the Federal University of Mato Grosso do Sul and lead author of the study. “That’s why the large aggregation we reported is so remarkable, as there had been no prior indication that such behavior was part of this genus’s natural history”. For ichthyologists, this represents a breakthrough in documenting behaviors of understudied riverine fish.
Climbing Against The Current
The footage shows the catfish scaling rocks between 3 and 13 feet tall, using an unusual locomotion technique. They straightened their fins, rocked side to side, and flicked their tails to push themselves upward. In some cases, they appeared to create a suction effect by pressing their bodies against the rock surface — similar to the strategy used by Andean climbing catfishes and stream loaches. Observers noted that during peak activity, individuals piled on top of one another, forming a moving carpet of fish across the rocks.
Remarkably, the climbing did not occur throughout the day. During hot afternoons, the fish sheltered in shaded pools at the base of the waterfall. Activity began in the cooler evening hours, just as the sun set, suggesting a potential link between environmental cues and the fish’s migration behavior. Scientists speculate that light, temperature, and water flow together triggered this sudden vertical climb.
A Mystery With A Likely Answer
Although the exact purpose of the climb remains under study, researchers strongly suspect reproduction as the driver. The timing of the event coincided with the beginning of the rainy season, when river levels rise dramatically following a long, severe drought. This hydrological shift may serve as a signal for spawning migrations. “All the signs point to this being a reproductive gathering,” said Marinho. “It happened right after a long and severe drought in the region, and the sudden rise in water levels seems to have triggered the species to spawn, bringing together a massive concentration of fish that had never been seen before.”
The climbing group was composed mainly of adults, both male and female, reinforcing the idea that the behavior is tied to upstream movement for reproduction. For now, the event remains one of the most dramatic and least understood collective behaviors ever recorded among small South American fishes.
Implications For River Ecosystems
The discovery underscores the ecological importance of maintaining free-flowing river systems. Smaller fish species are often neglected in migration studies, which tend to focus on commercially valuable species such as large catfish or migratory characins. Because events like this occur under narrow environmental conditions — such as seasonal flooding or drought recovery — they are easily overlooked by researchers.
Habitat fragmentation caused by hydroelectric dams and reservoir construction poses a significant threat. These projects disrupt natural flow patterns, reduce oxygen levels, and drown out rapids and waterfalls where climbing behaviors can occur. The team emphasized that protecting dynamic habitats is essential for preserving the life cycles of overlooked fish species. As Marinho and her colleagues noted, documenting such phenomena highlights the need for habitat conservation at a time when many river systems face mounting pressures.
Expanding The Scope Of Freshwater Research
For ichthyology, this event opens a window into a largely invisible dimension of river biodiversity. The Rhyacoglanis genus had not been associated with large-scale aggregations or vertical climbing behavior, leaving a gap in understanding of its natural history. By capturing this rare spectacle, scientists not only add to biological knowledge but also broaden awareness of the ecological roles played by small fish in South American rivers.
The findings encourage a reassessment of research priorities. If smaller species like the bumblebee catfish demonstrate complex migration strategies, their survival may be far more dependent on intact river systems than previously assumed. Such insights stress the value of continued monitoring and fieldwork in regions where environmental changes are accelerating.